x. Badger et al.( 2013)
Fig. 10. Boron( panel a, blue points) and alkenone( panel b, red points) pCO 2 reconstructions against carbon isotopes( panel c) from Badger et al.( 2013) for the period 13.5-13.8 Ma. Red lines added.
This study employs both boron and alkenone proxies to a Miocene geological section in Malta ranging from 12.5 to 14 Ma. Most of the data are in the 250-350 ppm range, with the boron giving somewhat lower estimates than the alkenones. The data seem to show a declining trend.
4. Synthesis
So let us return to the question: when was atmospheric pCO 2 last higher than 400 ppm? All records agree it was before the Pleistocene(> 2.6 Ma). The bulk of the data from the Pliocene epoch( 2.6-5 Ma), using a range of approaches and measurement techniques, are in the range 250-400 ppm and many records seem to indicate a declining trend through that time. We cannot conclude that pCO 2 was > 400 ppm in the Pliocene as a whole – if it was, it was only for short periods. Some of the alkenone